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Overview of study design. The main risk period was days 0-28 after receipt of a bivalent covid-19 mRNA booster as fourth dose. The total reference period consisted of ≥29 days after a third dose (up to the day before the fourth dose (ie, day 0 minus 1 day)) and the fourth dose (second booster). See supplementary figure S1 for the distribution of reference period and main risk period person time during the study period
